The present invention relates generally to clutches having two friction discs, whether these are clutches having two outputs, which are commonly called double clutches and in which each friction disc is rotationally fixed on a separate driven shaft, or whether they are single-output clutches which are commonly known as twin-disc clutches in which the two friction discs are rotationally fixed on the same driven shaft.
As is well known, a clutch of this kind, which is used particularly in vehicles and more particularly in agricultural tractors, is composed generally of two clutch assemblies each comprising a first plate known as the reaction plate and intended to be rotationally fastened to a first shaft, usually a driving shaft, a friction disc mounted for axial movement relative to the said reaction plate and intended to be rotationally fixed to another shaft, usually a driven shaft, a second plate known as the pressure plate and mounted for axial movement relative to the reaction plate and rotationally fastened thereto, and elastic means such as a spring or springs urging the pressure plate in the direction of a position of engagement in which the friction disc is clamped between the pressure plate and the reaction plate.
In order to reduce the axial dimensions of these elastic means, it has already been proposed to place them between the two pressure plates so that they form common elastic means disposed axially between the pressure plate of the first clutch assembly and the pressure plate of the second clutch assembly, and to make them in the form of a spring washer of the Belleville type.
However, a Belleville washer by itself is suitable only for relatively moderate loads.
Furthermore, taking into account the "saddleback" characteristic load curve of a Belleville washer, in the case of a double clutch the disengagement of one of the two friction discs inevitably leads to a reduction of the load applied to the other disc.
This may lead to the slipping of the other disc, and result in poor transmission of the torque applied to it and undue heating of the clutch assembly.
Conversely, if steps are taken to avoid this slipping by using a Belleville washer having adequate characteristics to ensure that a suitable load will always be applied in these circumstances to the other friction disc, the Belleville washer will then apply an excessive load to the friction discs when disengagement is not effected.
Furthermore, since the Belleville washer is common to both clutches, it must be capable of coping with a travel which double that of a single clutch. This leads to increased "saddlebacking" of the load characteristic of the Belleville washer. Consequently, in proportion as the friction discs wear, the point of this load characteristic which corresponds to conjoint engagement of the two clutches is displaced in the direction of high loads, with the risk that either of the clutches will be able to transmit without slip, a high torque capable of damaging the components served by the clutch.
At the same time, on declutching, i.e. when one of the clutches is disengaged, the load of the Belleville washer may attain a value which is so low that the Belleville washer will no longer be able to overcome the internal frictions which are inevitably present in a clutch of this kind, and consequently the clutch will only be able to return to the engaged position under bad conditions, despite the decrease of the force which effected its disengagement.
In order to overcome these disadvantages, and in addition to obtain generally reduced saddleback load characteristics, it has been proposed for two Belleville washers to be grouped together. In certain constructions of this type, the two Belleville washers are, as before, disposed conjointly between the two pressure plates.
As previously, this has the result that the disengagement of one of the friction discs will inevitably lead to a reduction of the load on the other disc, with the consequent risk of slipping.
In order to eliminate this disadvantage, either the total load of the two Belleville washers and therefore the maximum torque which can be transmitted without slip by the clutches in question must be limited, or the diameter and consequently the space required by these clutches must be increased.
It has however already been proposed to use two independent Belleville washers, which are separated from one another by an annular spacer fastened axially on the reaction plate of one of the clutch assemblies, while on the two axially opposite faces of the said spacer there are formed respectively in each case two support shoulders against which the outer peripheries of the Belleville washers bear.
However, the arrangement adopted in this construction, and as described in particular in French Pat. No. 1,120,930, implies in practice that in order to permit the various component parts of the clutch to be placed in position axially on the same side, the skirt which one of the reaction plates usually carries, in a generally axial arrangement, to constitute a cover in relation to the other of the said parts must necessarily be axially divided, which obviously complicates the construction and assembly of the whole arrangement.
In this arrangement, the annular spacer axially separating the two Belleville washers from one another belongs to the skirt, so that it is actually fastened axially on the said skirt, but since the support shoulders which the spacer carries for the Belleville washers are necessarily inside the space formed by the latter the spacer must obligatorily constitute a separate part from the skirt in order to permit the axial engagement of that one of the Belleville washers which is axially nearer the reaction plate to which the skirt belongs.
The present invention relates generally to an arrangement enabling this disadvantage to be overcome, and permit the unitary construction of the reaction plate skirt forming a cover, while enabling the axial mounting on one and the same side of the various parts in question.